1. Field of the Invention
The present invention relates to a method and a device for driving an AC type plasma display panel.
A plasma display panel (PDP) has two features of high speed and high resolution that are suitable for television sets and computer monitors. A PDP is used for a large screen display device. One of tasks about a PDP is to reduce a pseudo contour of an animation display.
2. Description of the Prior Art
For a gradation display using a PDP, a method is widely used that comprises the steps of replacing one frame with a plurality of subframes having weights of luminance, and setting on and off of light emission of each cell of a subframe. For example, cells are lighted only in subframes having weight 1 for gradation 1 and are lighted in subframes having weight 2 and in subframes having weight 8 for gradation 10, so that each of gradation levels corresponds to a combination of subframes to be lighted (this is called a subframe expression). Usually, the conversion from a frame into subframes is performed by using a conversion table that was made in advance. In the case of an interlace display, each of fields of a frame is made of plural subfields, and the lighting control is performed for each subfield. However, contents of the lighting control are similar to the case of a progressive display.
In a display by controlling lighting for each subframe, there is a problem of dynamic pseudo contours that can occur due to discrete light emission timing in a frame period. The dynamic pseudo contour is a phenomenon that an observer recognizes light and dark patterns that is not the display contents. Especially, the dynamic pseudo contour can occur easily when an image portion including pixels with similar gradation levels and having a gentle gradient of luminance moves in the screen. For example, in a scene where a man walks, the head portion of the man may generate the dynamic pseudo contour. In a static picture shown in FIG. 8A, areas of Gradation 2, Gradation 3, Gradation 4 and Gradation 5 are observed correctly, while a moving picture shown in FIG. 8B is observed as if a dark line exists at the boundary between the areas of the Gradation 3 and the Gradation 4. There is also a case where a moving picture is observed as if a light line exists depending on a movement of a line of sight.
FIGS. 9A-9C show a concept of a superposition method. The superposition method is used for reducing dynamic pseudo contours. In the superposition method, at least one group of two subframes having the same luminance weight is provided. FIG. 9A shows an example of providing Weight 4 in a structure having four subframes, in which a frame period is divided into four subframe periods Tsf1, Tsf2, Tsf3 and Tsf4t. Italic numerals in FIG. 9A show the weights. The cells in the display screen are classified into Group A and Group B as shown in FIG. 9B. A typical arrangement of cells is a checked pattern arrangement. In the gradations in which one of the two subframes having the same weight is lighted (i.e., the Gradation 4, the Gradation 5, the Gradation 6 and the Gradation 7 in the illustrated example), one subframe of the Weight 4 (the front side in the illustrated example) is lighted for cells of the Group A, while the other subframe is lighted for cells of the Group B. For example, when a display changes from the Gradation 3 to the Gradation 4 as shown in FIG. 9C, a light emission period becomes longer in cells of the Group B than in cells of the Group A. On the contrary, the light emission period becomes longer in cells of the Group A than in cells of the Group B when the display changes from the Gradation 4 to the Gradation 3. In this way, the light emission period is different between the Group A and the Group B despite of the same gradient of gradation. Therefore, the dynamic pseudo contour has a pattern in which a light portion and a dark portion interchange with each other for every cell as shown in FIG. 10. This pattern is a fine pattern that cannot be recognized in a usual observation. Namely, in the superposition method, the luminance is equalized by dispersing the light and dark for gradation portions having a tendency to generate the dynamic pseudo contour. Thus, the dynamic pseudo contour becomes inconspicuous visually.
However, in the conventional driving method using the superposition method, a check pattern noise can occur as shown in FIG. 11. For example, if the area of the uniform luminance of the Gradation 4 is spread widely in the display screen when performing the above-mentioned superposition process of the example shown in FIG. 9, light emission becomes a check pattern in the whole of the area. In a moving picture in which a wide area of the check pattern moves, an observer who traces the movement may recognize a luminance difference between the Group A and the Group B. Namely, if the area in which the luminance is equalized by the superposition process is wide, since the Group A and the Group B are distributed periodically, human eyes can observe the light and dark patterns even after the luminance is equalized. Thus, the image quality is deteriorated.